1. Field of the Invention
The present invention relates to a strobe apparatus for a still video camera having an image pickup device in which the color temperature of a strobe light is controlled so as to result in a natural color image, even with images which have a large step (or incremental) change in their color.
2. Description of the Related Art
In a conventional still video camera, a white light balance is adjusted so that a white object, when photographed, is reproduced as a white image, based on light reflected from the object, regardless of the color temperature of an illumination light used to illuminate the object. For instance, in a known still video camera having a strobe apparatus (electronic flash), the white balance adjustment is carried out by adjusting the gain of color difference signals (R-Y, B-Y) of an object image, etc., output from a solid-state image pickup device. In the situation where a strobe apparatus is activated to emit strobe light, the white balance is controlled in accordance with a predetermined color temperature of the strobe light.
However, if the color temperature of strobe light is different from the color temperature of ambient light, there is a possibility that the reproduced image will have an unnatural color. To prevent this, it has been proposed in Japanese Patent Application No. 5-235518 by the assignee of the present application that a color temperature of a strobe light emitted from a xenon tube (light emitting tube) be controlled to be substantially identical to a color temperature of the ambient light, by means of a color temperature conversion filter provided in front of a xenon tube.
However, the color temperature of the strobe light emitted from the light emitting tube or the conversion power of the color temperature conversion filter tends to vary with time, or to vary spontaneously during the emission of light. Consequently, if the emission time or the conversion power of the color temperature conversion filter is constantly controlled with respect to the color temperature of the ambient light in a predetermined mode, there can be an error in the color temperature of the strobe light incident upon the object, and thus, the resultant color temperature varies. Moreover, since the white balance in an image pickup system is effected so that the color temperature of the strobe light is identical to the color temperature of the ambient light, if there is an error of the color temperature of the strobe light or a variation in the color temperature during the emission, no white balance of the object image can be achieved.
If there is a difference in the quantity of the light between the light emitting tubes due to irregular emission characteristics of the light emitting tubes or irregular light receiving sensitivities of the photometers, the resultant color temperature can be wrong.
In this arrangement, since two emissions of the strobe light occurs for one photograph, the quantity of electric charges to be discharged from the trigger capacitor in the strobe apparatus is increased. Moreover, since it is necessary to charge the trigger capacitor in order to effect the second emission after the first emission is completed, it takes a long time to control the emission of the strobe light. In addition to the foregoing, if two xenon tubes are used, a range in which the color temperature of the resulting strobe light can be correctly controlled is reduced due to a possible deviation or difference in the illumination area between the two strobe lights.
However, the color temperature of the xenon tube increases as the emission time decreases, and accordingly, no precise control of the color temperature can be effected by the above-mentioned structure.
However, if there is a difference in the color temperature between the strobe light and the ambient light of the object to be photographed, there is a possibility that an unnatural color of an object image will be reproduced.